1. Field of the Invention
The present invention relates to solid state imaging devices being manufactured in a CMOS- or MOS-technology. More particularly, an amplifying circuit and a method for eliminating fixed pattern noise in the output signal of a pixel or of an image sensor are disclosed.
2. Description of Related Technology
Solid state image sensors are well known. Commonly solid state image sensors are implemented in a CCD-technology or in a CMOS- or MOS-technology. Solid state image sensors find a widespread use in camera systems. In this embodiment a matrix of pixels comprising light sensitive elements constitutes an image sensor, which is mounted in the camera system. The signal of said matrix is measured and multiplexed to a so-called video-signal.
CCD-based camera systems have less noise fluctuations in the image compared to CMOS- or MOS-based camera systems. Therefore CCD-based camera systems are nowadays preferred in applications wherein a high image quality is required such as video or still camera applications. Due to the further miniaturization of the CMOS electronics technology, it is possible to realize complex CMOS- or MOS-based pixels as small as CCD-based pixels. It is a further advantage of CMOS- or MOS-based pixels that CMOS is a technology being offered by most foundries whereas CCD-technology is rarely offered and a more complex and expensive one.
Of the image sensors implemented in a CMOS- or MOS-technology, CMOS or MOS image sensors with passive pixels and CMOS or MOS image sensors with active pixels are distinguished. An active pixel is configured with means integrated in the pixel to amplify the charge that is collected on the light sensitive element. Passive pixels do not have said means and require a charge-sensitive amplifier that is not integrated in the pixel and is connected with a long line towards the pixel. For this reason, active pixel image sensors are potentially less sensitive to noise fluctuations than passive pixels. Due to the additional electronics in the active pixel, an active pixel image sensor may be equipped to execute more sophisticated functions, which can be advantageous for the performance of the camera system. Said functions can include filtering, operation at higher speed or operation in more extreme illumination conditions. It remains however a main drawback of active pixel CMOS or MOS image sensors, hampering their use in applications requiring a high image quality, that their output signal has an additional non-uniformity caused by the statistical spread on the characteristics of the electronic components composing the active pixel. An example of such characteristic being subject to manufacturing process variations is the threshold voltage of MOS transistors integrated in the pixel. If no precautions are taken, this non-uniformity, called fixed pattern noise or FPN, is seen as a "snow-like" shade over the image being taken with a CMOS or MOS image sensor with active pixels.
Document U.S. Pat. No. 4,274,113 discloses a solid state imaging device having a sensor portion and a signal processing circuit. Said signal processing circuit comprises means to eliminate fixed pattern noise. In this device the charge collected on a passive pixel is measured on a common charge amplifier. The pixel is sampled two times consecutively, and the difference of charges is constituting the signal. The first sampling is taken with the pixel not connected to the output, the second sampling is taken with the pixels connected to the output for a short time, so that the charge is integrated in the output. This method to eliminate FPN however is not suited for active pixels which have not a charge output, but which have a voltage output or current output or similar. Also, as disclosed in the preferred embodiment of U.S. Pat. No. 4,274,113, reading charge over a large bus with high capacitance deteriorates the temporal noise in the final sensor image.
Document U.S. Pat. No. 4,809,074 discloses a solid state imager having a signal processing circuit for reducing noise, in particular FPN. Each pixel has two switches, which makes it possible to read a pixel charge with less noise and FPN than in pixels with only one switch. The FPN cancellation is performed by reading twice the charge on the output node. Again, the method disclosed in this patent is not suited for active pixel imaging sensors.
The patent application Ser. No. WO92/16999 discloses a circuit for minimizing the variation in characteristics across different parts of an imaging device caused by manufacturing process variations between a plurality of inverting amplifiers in said imaging device. Said circuit comprises at least one transistor connected in series with a power supply terminal on each of the inverting amplifiers so as to provide a new reference level for each inverting amplifier whereby the switching threshold of the inverting amplifier is controllable by a voltage applied to a control input connection of said transistor. This method applies only for an imaging device with passive pixels.
Document EP-A-0665685 discloses an active pixel image sensor. This patent describes a method to cancel non-uniformity of the pixel response. This method is based on the fact that all pixels outputs are fed through switches and column/row buses to a common output, without passing trough intermediate (column) buffers. This method is viable, but it requires the output stage of every individual pixel to be powerful enough to drive the buses/output lines at a high readout speed. The proposed method furthermore adds additional fixed pattern noise by the multiplexing structures.